Fas ligand is targeted to secretory lysosomes via a proline-rich domain inits cytoplasmic tail

Fas ligand (FasL) induces apoptosis through its cell surface receptor Fas. T lymphocytes and natural killer cells sort newly synthesised FasL to secre tory lysosomes but, in cell types with conventional lysosomes, FasL appears directly on the plasma membrane. Here, we define a proline-rich domain (PR D) in the cytoplasmic tail of FasL that is responsible for sorting FasL to secretory lysosomes. Deletion of this PRD results in cell surface expressio n of FasL in cells with secretory lysosomes. Positively charged residues fl anking the PRD are crucial to the sorting motif and changing the charge of these residues causes missorting to the plasma membrane. In cells with conv entional lysosomes, this motif is not recognised and FasL is expressed at t he plasma membrane. The FasL PRD is not required for endocytosis in any cel l type, as deletion mutants lacking this motif are endocytosed efficiently to the lysosomal compartment. Endogenous FasL cannot internalise extracellu lar antibody, demonstrating that FasL does not transit the plasma membrane en route to the secretory lysosomes. We propose that an interaction of the PRD of FasL with an SH3-domain-containing protein, enables direct sorting o f FasL from the Golgi to secretory lysosomes.